Industrial lignin products are dark brown solids that are obtained as co-products of the manufacturing of cellulose pulp for paper and other recovery processes. As of 2010, the pulp and paper industry alone produced an estimated 50 million tons of extracted lignin. Only 2% of the production (one million ton) was commercially used for industrial products such as dispersing or binding agents; the rest was burnt as low-value fuel.
Chemically, lignin contains varied levels of aliphatic hydroxyl groups, phenolic hydroxyl groups and carboxyl groups that, in principle, can react with isocyanates to produce polyurethane materials. In order to utilize lignin for making polyurethanes the solid lignin needs to be converted into liquid. There are two major technologies to serve this purpose. One is to dissolve chemically unmodified lignin in polyols and the other is to modify lignin chemically with alkoxylation process, that is, to make lignin-based polyether polyols.
For dissolving lignin into polyols or polyol blends, both polyether polyols and polyester polyols have been used according to the available literature. One of the most commonly used polyol is polyethylene glycol (PEG) of molecular weight less than 1000 Dalton. In the blending process, aggressive mechanical mixing, for example, high-shear mixing, is used to obtain homogenous lignin/polyol liquids. Such lignin-based polyols have high acidity.
The problem with high acidity polyols has been the conversion of isocyanate to urethane decreases when reacted with increasing amount of lignin in the blends with polycaprolactone polyol of different molecular weight. Moreover, the reactivity between isocyanate and the lignin/polyol blend is dependent on the lignin type as well.
U.S. Pat. No. 6,025,452 teaches that the reactivity of the lignin/polyol mixture is controlled by the sodium concentration and the preferred sodium concentration is less than 1% by weight of the lignin.
United States Patent Application No. US2015/0259369A1 relates to a system and method for preparing chemically modified lignin.
Reactivity of polyol with isocyanates is of importance for the polyurethane rigid foam industry. Other than formulation factors, for example, catalyst type and concentration, the polyol reactivity is correlated with the types of hydroxyl groups, which include primary hydroxyl, stereo-hindered hydroxyl, aliphatic hydroxyl, or phenolic hydroxyl groups and the acidity of the polyol. The commercial acid number values for industrial polyols are less than 2 mg KOH/g in order to minimize the negative impact on reactivity and to ensure processing latitude in foam production.
Alkoxylation may be advantageous in converting hydroxyl and carboxyl groups at different locations in lignin molecules to (methyl) hydroxyethyl end-groups. In a typical alkoxylation process, lignin may be reacted with propylene oxide or ethylene oxide, or in a mixture form, with an alkali catalyst under high pressure and elevated temperature conditions. In an ideal alkoxylation process, the hydroxyl groups present in lignin molecules can be converted to 1-methyl hydroxyethyl groups or hydroxyethyl groups uniformly, which would improve the reactivity of the resultant lignin-based polyol. Also, by altering the ratio of lignin/propylene oxide or ethylene oxide, lignin polyol may be made with desired hydroxyl number and polyol viscosity.
Alkoxylation process is basically converting lignin into lignin-based polyether polyols that are similar to those conventional polyether polyols. Polyurethane rigid foams are also obtained from these lignin polyols with good physical properties.
The disadvantage of lignin alkoxylation process is the difficulty of handling toxic propylene oxide and ethylene oxide. Another disadvantage is that often times low polyol viscosity can be achieved only when large amount of propylene oxide or ethylene oxide is used. This significantly reduces the bio-based content in the final polyol and the advantage of using lignin as a raw material is lost.
Polyurethane rigid foams for thermal insulation applications are produced with aromatic polyester polyols as one of the key components. Aromatic polyester polyol is generally produced by esterification/transesterification reaction of aromatic acid source with glycols, for example, diethylene glycol, branch molecules, for example glycerin and pentaerythitol, and modifiers for example, fatty acid source. The lignin molecule contains varied kinds of hydroxyl groups and small amount of carboxylic acid species, which gives the potential as an alcohol-functional species for aromatic polyester polyol production. However, it has been an industrial problem that the reactant aromatic polyester polyols end up with the acid number higher than the acceptable range.